Simulations of solvent effects on excited state dynamics of p-DAPA, a red single benzene-based fluorophore.

Fluorescence-based techniques are widely used in basic science and medicine for detecting molecules and probing chemical reactions in vitro and in vivo. Red fluorophores are especially of interest due to their potential applicability for deep-tissue and whole-body imaging with low background interference. The smallest red fluorophore is the benzene-based para-diacetylphenylenediamine (p-DAPA), which has a fluorescence quantum yield of 0.06 and high brightness. However, the effect of solvent type on the photophysical properties of p-DAPA has not been extensively explored. We have employed classical force field dynamics and ab initio Born-Oppenheimer molecular dynamics calculations in an explicit solvent to explore the ground and excited state potential energy surfaces of p-DAPA. Our results show that p-DAPA exhibits stronger fluorescence in nonpolar solvents due to the appearance of additional nonradiative decay pathways in polar solvents. The type of solvent modulates the type and speed of decay pathways observed, while the overall character of the nonradiative state is maintained. This work provides insight into how p-DAPA might be adapted for biological imaging and new solvent environments.